matched fets

[frank05330]

hey does anyone know of anywhere i could get matched fets for a phase 45? smallbear sell them but i dont think they come matched. and if i was to match the fets myself, how many should i order so that i have a fair chance of finding a good pair?
thanks

[Fuzz]

hey does anyone know of anywhere i could get matched fets for a phase 45? smallbear sell them but i dont think they come matched. and if i was to match the fets myself, how many should i order so that i have a fair chance of finding a good pair?
thanks

Hi Frank!
Why don't you trust in SmallBear promises? I think they're cool guys, at least for what I can read around the web (never ordered, I'm from Italy). Anyhow, when I made my Phase45 (one of my first diy effects) I just bought two unmatched BF245A, and they worked great, even if they were not exact equivalent with the original! Maybe I've been lucky, maybe it could have been sounding better with a pair of matched ones...but we're talking about modern stuff, not 30 years old germanium trannies, so you won't have to buy tons of them to find a pair of matched...I mean, ten could be too many!

Bye!
Fuzz

[petemoore]

Check out GEO for reads on how to get Jfet Phasers working.

[Mark Hammer]

The choice of FET itself will determine the amount of usable sweep.  If the FET can only vary its resistance when the gate voltage falls between values A and B, and the A-to-B range is only a small portion of, or only overlaps minimally with, the voltage swing range of the LFO, then there will be little sweep.  It's a bit like the asking how much you like the buddies you have on the job if your job consists of being the solitary night watchman and you come on shift 15 minutes before everyone else leaves and stick around for 15 minutes after they arrive in the morning.  Not much chance to mingle, eh?

The usable range of the 2N5952 is optimal for the Phase 45 and Phase 90.  Other FETs can sub as well, but from what I understand those may involve picking them from the range of possible values that specific FET model can fall within.  2N5952's tend to work pretty much out of the box, though matching may improve that.

The matching itself is intended to assure that all FETs continue their linear resistance change simultaneously.  Although the sweep continues through the entire cycle, should the FETs be poorly matched, one or more of them will throw its hands up and declare "That's it, mates.  I'm tapped.!", with the result being that there may not be as much *cumulative* phase shift in certain parts of the spectrum at the extremes of the sweep cycle as you might want.  Of course, the overall intensity of the effect will also depend on things like:
whether the dry and wet signal levels are perfectly matched at the mixing stage
whether you have any regeneration or not and how much
whether the instrument signal feeding the phaseshifter even HAS any frequency content to be notched at the extreme ends of the sweep cycle
how many stages you're working with

In the case of the P45, that last one is pretty central.  If you had 10 stages and one crapped out on you 80% of the way into an upward sweep, you'd still have a pretty intense phase shifting effect.  If 4 of 10 simply couldn't keep up (because of poor matching), then you'd be depriving yourself of maximum phasing possible from the design.  If you only have TWO phase shifting stages, as in the P45, the difference between the most and least phasing sound you can get is certainly affected by having one FET unable to keep up with the other, but on the other hand the effect is pretty subtle anyways, so you may not even notice it that much.